Many of today's vehicle brake systems are hydraulic brake systems including a brake disk and friction pads adapted to be pressed against the brake disk by hydraulic cylinders. But with the introduction of new brake control systems such as anti-lock brake systems (ABS), electric brake systems, which require no hydraulic circuits, are receiving attention these days.
Typically, electric brake systems have a linear motion actuator including a rotary shaft to which the rotation of an electric motor is transmitted, and a linear motion mechanism for converting the rotation of the rotary shaft to a linear motion of a linear motion member. The linear motion actuator applies an axial load to a friction pad to press the friction pad against the brake disk, thereby generating a braking force. In order to control the braking force to a desired magnitude, a load sensor unit is mounted in many of such electric brake systems for detecting the magnitude of the axial load applied to the object. For improved response of the electric brake system, it is preferable to use a load sensor unit which can detect a load applied with a minimum possible movement of its moving part or parts.
A load sensor unit for use in a linear motion actuator which satisfies this requirement is disclosed e.g. in the below-identified Patent document 1. This load sensor unit includes an opposed pair of annular presser plates, piezoelectric crystal elements disposed between the presser plates, an insulating plate electrically insulating the piezoelectric crystal elements from one of the opposed pair of presser plates, and a lead wire through which voltage generated by the piezoelectric crystal elements are taken out. When an axial load is applied to this load sensor unit, the piezoelectric crystal elements generate a voltage corresponding to the load applied. It is thus possible to detect the axial load applied by measuring the voltage generated. Since the presser plates are moved very little relative to each other due to deformation of the piezoelectric crystal elements, this sensor unit will never deteriorate response of an electric brake system if mounted in the electric brake system.
But since the load sensor unit disclosed in Patent document 1 is designed such that an axial load applied to the sensor unit directly acts on the piezoelectric crystal elements, if the axial load is an impulsive load or a shear load, one or more of the piezoelectric crystal elements may crack or chip. This load sensor unit is therefore not sufficiently durable.
Under these circumstances, the inventors of the present application attempted to develop an improved load sensor unit which is sufficiently durable, and is capable of detecting loads with a minimum displacement of the moving parts of the sensor unit. As a result, the inventors proposed a magnetic load sensor unit including a flange member configured to be deflected when the reaction force to an axial force applied by the linear motion actuator to an object is applied to the sensor unit, a magnetic target which generates magnetic fields, and a magnetic sensor arranged such that when the flange member is deflected by the reaction force, the position of the magnetic sensor relative to the flange member changes.
Since this magnetic load sensor unit is configured such that the flange member is deflected when the reaction force to an axial force applied by the linear motion actuator to the object is applied to the sensor unit, the relative position between the magnetic target and the magnetic sensor changes due to the deflection of the flange member, and the output signal of the magnetic sensor changes corresponding to the change in relative position, it is possible to detect the magnitude of the axial load based on the output signal of the magnetic sensor. Since this magnetic load sensor unit is configured to detect the above-described axial load based on a change in relative position between the magnetic target and the magnetic sensor, which are kept out of contact with each other, this sensor unit is less likely to malfunction when impulsive loads or shear loads are applied thereto, and is thus sufficiently durable.